Apparatus and method for the analysis of atomic and molecular elements by wavelength dispersive x-ray spectrometric devices

ABSTRACT

In an apparatus and a method for the analysis of atomic or molecular elements contained in a sample by wavelength dispersive X-ray spectrometry, wherein primary x ray or electron radiation is directed onto the sample whereby fluorescence radiation is emitted from the sample, the fluorescence radiation is directed onto a mirror or focussing device consisting of a multi-layer structure including pairs of layers of which one layer of a pair consists of lanthanum and the other consists of carbon and the fluorescence radiation is reflected from the mirror or focussing device onto an analysis detector for the analysis of the atomic or molecular elements contained in the sample.

BACKGROUND OF THE INVENTION

[0001] The invention relates to an apparatus for the analysis of atomicand/or molecular elements by wavelength dispersive X-ray spectrometricdevices comprising at least one reflection—or focussing device includinga multi-layer structure, particularly an apparatus wherein fluorescencerays generated by a sample to be analyzed when subjected to incidentprimary X-ray or electron radiation are directed onto a mirror orfocussing device before reaching a measuring or analysis detector. Themirror or focussing device is formed by a multi-layer structurecomprising layer pairs each including a first layer element formed bycarbon or scandium. The invention also resides in an analysis methodemploying such apparatus.

[0002] Apparatus and methods of this type are known for example from DEOS 199 26 056. They are used in scientific analyses but also inindustrial applications for the detection of atomic and/or molecularelements in various areas for example when impurities or disturbancespresent in examples in only small amounts are to be detected oranalyzed.

[0003] In that case, X-ray or electron beams from any type of X-ray orelectron source are directed onto a sample whereby, among others,fluorescence rays are returned from the sample which are induced by theincident X-rays by a well-known physical processes. These fluorescencerays are directed onto a suitable crystal where they are reflected andthen directed onto a measuring and analysis arrangement for example inthe form of a fluorescence radiation-selective detector. The crystalsact as analyzers. These crystals which can be artificial crystals. mayconsist of thin multiple alternate layers of two or more materials withdifferent X-ray optical properties. In connection with the aboveexample, the incident fluorescent rays are reflected from these layersbut only that part of the fluorescent rays for which the Bragg equation.

nλ=2d sin θ

[0004] is fulfilled,

[0005] Herein:

λ(nm)=1.24/E(keV)

[0006] wherein n is a natural number (n=1, 2, 3, 4 . . . )

[0007] λ=the wavelength of the x rays, that is,

[0008] d=the periodicity (lattice parameter) of the analyzer crystal,

[0009] 2θ=infraction angle, and

[0010] E=energy of the X-rays.

[0011] Taking into consideration the effect of the refraction, which isvery small for X-rays, results in an equation which is modified from thefirst equation whereby from the set angles θ and the lattice parameter dof the analyzer the wavelength of the reflected X-rays can be determinedfrom the first equation or the modification thereof. By varying theangle therefore the wavelength of the reflected rays, that is in theabove example the fluorescence rays, can be selected in a controlledmanner.

[0012] The big advantage of the artificial crystals which consist ofmany regularly changing layers—called in this connection alsomulti-layer arrangement—is that the materials, of which the multi-layerarrangement consists, can be selected and optimized for best results.This is an essential advantage of the manufactured multi-layerarrangement as compared to natural crystals.

[0013] The intensity of the reflected radiation depends greatly on thematerials used for the multi-layer. In addition, the lattice parametersof the multi-layer can be modified within a larger range than it ispossible with natural crystals.

[0014] It is a particular advantage of the multi-layer analyzer that itfacilitates the analysis of light elements with uniform intensity andwithout health-endangering side effects.

[0015] In many cases so far the multi-layer structure or, respectively,the individual layers of the multi-layer structure, have been adaptedspecifically to the element expected to be analyzed. For the specialcase of an energy range of 100 to 180 eV, that is, particularly for thedetection of beryllium and boron, in the past, tungsten-carbonmulti-layers have been used. Newer developments of lanthanum boroncarbide multi-layer arrangements further improve the detection limit ofboron, see DE OS 199 26056 referred to earlier. For the detection ofcarbon at energies of about 277 eV, multi-layer arrangements ofvanadium-, or respectively, nickel-carbon have been used, see also U.S.Pat. No. 4,785,470.

[0016] Multi-layer arrangements which are utilized mainly in x-rayspectrometers for the detection of boron and beryllium (Mo—B₄C) or whichprovide the best detection limits (La—B₄C) have only a smallreflectivity of 3 to 0.9% for carbon. The reason herefor is that, in thelight layer material B₄C, boron is contained which is an element whoseabsorption edge is at an energy below the carbon emission line of 277eV. Therefore the carbon radiation of multi-layer structures includingB₄C is highly absorbed. In contrast, the multi-layers V—C and Cr—C,which are optimized for the carbon detection are not well suited for thedetection of the light elements beryllium and boron, because theirreflectivity in comparison with lanthanum containing multi-layerarrangements is lower by more than the factor 2. This low reflectivityis insufficient for the detection of the light element B and Be, mainlybecause the light elements have a substantially smaller fluorescenceyield than the heavier elements so that the expected count rates arecomparatively low anyhow.

[0017] For the detection of the light elements beryllium to carbon,which have their highest reflectivity at energies of 108 183 and 277 eV,so far at least two different optimized multi-layer arrangements arenecessary.

[0018] It is therefore the object of the present invention to provide anapparatus and a method with which a very much improved x-ray analysisfor the detection of beryllium, boron and carbon is possible, andwherein the apparatus as well as the method are to be set up andoperated in a simple manner utilizing means known from the state of theart, so that present analysis apparatus and methods can be utilizedwithout major changes whereby the apparatus and the method can beestablished relatively inexpensively and operated in a simple manner andat low costs by research institutions and industrial installations.

SUMMARY OF THE INVENTION

[0019] In an apparatus and a method for the analysis of atomic ormolecular elements contained in a sample by wavelength dispersive X-rayspectrometry, wherein primary x ray or electron radiation is directedonto the sample whereby fluorescence radiation is emitted from thesample, the fluorescence radiation is directed onto a mirror orfocussing device consisting of a multi-layer structure including pairsof layers of which one layer of a pair consists of lanthanum and theother consists of carbon and the fluorescence radiation is reflectedfrom the mirror or focussing device onto an analysis detector for theanalysis of the atomic or molecular elements contained in the sample.

[0020] The advantage of the apparatus and method according to theinvention resides essentially in the fact that the combination oflanthanum and carbon according to the invention for the layer elementsforming the layer pair in multi-layer arrangements for wavelengthdispersive analyzers of x-rays in the energy range of about 108 eV forthe detection of boron and in the range of about 277 eV are used for thedetection of carbon. The particularly advantageous x-ray opticalproperties of the layer elements of lanthanum and carbon result, incomparison with the best conventional analyzers referred to above withoptimized multi-layer arrangements, in reflectivities which arecomparable for the detection of beryllium, slightly lower than La—B₄Cfor boron and also slightly lower for carbon. La—C is the only materialcombination which has high reflectivity for all three elements Be, B andC at the same time. Furthermore, LaC multi-layer arrangements, forexample as La—B₄C— multi-layer structures, provide during borondetection for a substantially improved suppression of the oxygen K— aswell as the silicon-L lines because of the use of lanthanum as heavymaterial (reflector) . This suppression is also improved for theC-radiation in comparison with the special mirrors of Ni—C, orrespectively, V—V layer pairs for the detection of carbon.

[0021] In accordance with a preferred embodiment of the apparatus, themulti-layer arrangement comprises 1 to 100 layer pairs that is 2-200individual layers. The number of layers or layer pairs which areselected for the formation of a certain multi-layer structure dependsessentially on the intended measuring task and the type and amount ofimpurities expected to be present in the sample to be examined.

[0022] It is particularly advantageous if the multi-layer structurecomprises 40-50 layer pairs that is 80 to 100 individual layers.

[0023] In a basic version of the apparatus, the thickness of therespective multi-layer structure is constant. However, it is alsopossible that the thickness of one layer of each multi-layer pair isdifferent from the other layer of the multi-layer pair.

[0024] Basically, it is made sure in the embodiment described above thata parallel fluorescence beam is reflected at the complete surface of themulti-layer structure with maximum intensity.

[0025] In another advantageous embodiment, the thickness of therespective multi-layer varies over its extent whereby it is ensured thatnon-parallel fluorescence rays reaching the multi-layer structure underdifferent incident angles are reflected for the desired wavelength overthe full surface of the multi-layer structure with maximum intensity.The different incident angles θ are compensated for in accordance withthe equation given earlier or the corrected modification thereof by avariation of the lattice parameter d, so that λ remains constant.

[0026] Preferably, the apparatus is so modified that the multi-layerstructure is curved or, in another advantageous embodiment, themulti-layer structure is disposed on a support surface (substrate). Thishowever is possible in connection with all embodiments of themulti-layer structure. In this way, it is ensured that a nonparallelfluorescence beam which reaches the multi-layer structure at differentpoints under different incident angles, can be influenced so that forexample a divergent fluorescence beam can be converted into a parallelor a focussed fluorescence beam. It may also be advantageous if thethicknesses of the individual layers of the multi-layer structure aredifferent, that is, if their thicknesses vary in a suitable manner sothat the multi-layer structure reflects the fluorescence radiation ofthe desired wavelength which reaches the multi-layer structure underdifferent angles with maximum intensity over the whole area.

[0027] The multi-layer structure may comprise a combination wherein oneof the layers of the multi-layer pair has a uniform thickness whereasthe other layer has a varying thickness.

[0028] Preferably, the layer thickness is in the area of 1 to 20 nm.Examinations have revealed that with such layer thicknesses the highestreflectivity and the best resolution of the multi-layer structure can beachieved.

[0029] It is particularly advantageous if the layer has a thickness ofabout 8 nm possibly with a layer thickness ratio of Γ=0.4 with 100 layerpairs if applicable.

[0030] The method for the analysis of atomic and/or molecular elementsby means of wavelength dispersive X-ray spectrometric apparatuscomprising at least one mirror or focussing arrangement including amulti-layer structure onto which the primary X-ray or electron radiationis directed, wherein particularly fluorescence radiation induced by theincident primary X-ray and electron radiation is directed onto themirror or focussing arrangement before the radiation is directed onto ameasuring or analyzing detector and wherein the mirror or focussingarrangement includes at least one layer pair of a multi-layer structureand a first layer element of the layer pair consists of lanthanum andthe second layer of the layer pair consists of carbon. 109 eV (BE) 133eV (B) 277 eV (C) La-C 22.3% 37.2% 37.4% Mo-B₄C 18.3% 36.4% 3.0% La-B₄C23.7% 62.6% 0.9% V-C 4.2% 14.6% 45.1% Cr-C 6.1% 18.1% 45.2%

[0031] Theoretical reflectivity of multi-layers (d=8 nm, Γ=0.4 and layerpairs) optimized for the Be, B— and C detection.

[0032] Generally, the method according to the invention has the sameadvantages as they have been described for the apparatus according tothe invention. Reference is therefore made to the advantages givenearlier for the apparatus.

[0033] The invention will now be described for a particular embodimentwith reference to the attached schematic drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0034]FIG. 1 shows radiation from an X-ray source reaching a sample fromwhich it is reflected onto a multi-layer mirror and then onto ameasuring or analysis apparatus (detector),

[0035]FIG. 2 shows incident rays (in the example fluorescence radiation)reaching the multi-layer structure of the apparatus according to theinvention, which multi-layer structure is disposed on a substrate.

[0036]FIG. 3 is a representation of the reflectivity of a La—Cmulti-layer structure consisting of 100 periods with d=8 nm and a layerthickness ratio Γ=0.4 as a function of the angle for the boronradiation, and

[0037]FIG. 4 is a representation of the reflectivity of a La—Cmulti-layer structure consisting of 100 periods with d=8 nm and a layerthickness ratio Γ=0.4 as a function of the angle for carbon radiation(277 eV).

DESCRIPTION OF A PARTICULAR EMBODIMENT

[0038] First reference is made to FIG. 1, which schematically shows anapparatus 10 for the analysis of atomic and/or molecular elements inaccordance with the invention as it may be used with some variations formany applications.

[0039] From an X-ray or electron source, which is not shown in thefigure, primary X-rays or electron rays 15 are emitted and are directedonto a sample 14 for example in the form of a silicon wafer fordetecting impurities in the silicon wafer on or near the surfacethereof. Because of well-known physical phenomena, fluorescence rays 16are generated as reflected rays, which include information concerningthe type of additional atomic and/or molecular elements contained in thesample 14. The fluorescence rays 16 are directed onto a mirror orfocussing device 11, which in the example of FIG. 1 consists of twomirror or focussing devices 11 formed each by a multi-layer structure12. In another embodiment of the apparatus 10, the focussing deviceconsists only of a mirror or of a focussing structure. The fluorescencerays 16′ or 16″ reflected from the mirror or focussing device aredirected onto a measuring or analysis arrangement (detector) 17 by meansof which, in a known manner, quantitative and qualitative informationconcerning the type of the atomic and/or molecular elements present on,or in the material of, the sample 14 can be obtained.

[0040]FIG. 2 shows a section of the mirror and focussing device 11,which represents the actual multi-layer structure 12 disposed in thiscase on a substrate 19.

[0041] The individual layers 13 ₁. . . 13 _(n) form with the pluralityof all the pairs 13 ₁, 13 ₂; 13 ₃, 13 ₄; 13 ₅, 13 ₆ etc. the completemulti-layer structure 12. The layers of each layer pair consist of a Lalayer and a C layer and a metal oxide layer or a metal nitride layer(second layer element). The order of the layer in each layer pair may beselected as desired. The first layer 13 ₁ of a layer pair may forexample be lanthanum and the second layer 13 ₂ may be carbon. However,the first layer 13 ₁ may also be carbon and the second layer 13 ₂ may belanthanum. The incident rays or beam, in the example fluorescence rays16, are reflected at the interfaces of the different layer pairs and thereflected rays or beams 16′ leaving the mirror or focussing structure 11are directed onto a second mirror or focussing structure 11, see FIG. 1.Reflected from there, they reach the measuring or analysis device 17.However, they may also reach the measuring and analysis device 17directly without being reflected from a second mirror or focussingstructure 11.

[0042] From FIG. 3, it is apparent that the multi-layer(La—C-multi-layer) of the FIGS. 3 (for the boron analysis) and 4 (forthe carbon analysis) and the table presented earlier provide for obviousadvantages. FIGS. 3 and 4 show the theoretical reflectivities of anideal multi-layer with a layer thickness of Γ=0.4 and 100 layer pairs.The La—C multi-layer structure has for beryllium and boron a lowerreflectivity only with respect to a La—B₄C multi-layer structure but asubstantially higher reflectivity than the Mo—B₄C commercially availableso far for those elements. In comparison with this layer structure,which is optimal for the Be— and B detection, however the La—Cmulti-layer structure has a high reflectivity also for carbon which isalmost as good as the multi-layer structure V—C or respectively, Cr—Coptimized for the C detection. A La—C multi-layer structure can therefordetect the elements Be—B and C with a single analyzer in a way notachievable so far. As a result, for applications in which Be or B and Care to be detected, an analyzer crystal is not needed.

[0043] The method according to the invention is performed in accordancewith the ray path from the x-ray or electron beam source (not shown) tothe measuring or analyzing device (detector) 17 as shown above inconnection with FIG. 1, which shows the apparatus 10 according to theinvention in an exemplary manner.

What is claimed is:
 1. An apparatus for the analysis of atomic andmolecular elements by wavelengths dispersive X-ray spectrometric devicescomprising a mirror or focussing structure including a multilayerstructure onto which fluorescence rays emanating from a sample to betested by primary X-ray or electron radiation, which is directed ontosaid sample, are directed and from which they are reflected, and ameasuring or analysis detector onto which said reflected fluorescencerays are directed for the analysis of the atomic and molecular elementsin said sample, said multi-layer structure comprising at least one pairof layer elements of which a first layer element of the pair is formedby lanthanum and the second is formed by carbon.
 2. An apparatusaccording to claim 1, wherein said multi-layer structure includes 1-100layer pairs.
 3. An apparatus according to claim 1, wherein saidmulti-layer structure includes 40-50 layer pairs.
 4. An apparatusaccording to claim 1, wherein said multi-layer structure has a uniformthickness.
 5. An apparatus according to claim 1, wherein saidmulti-layer structure has a thickness which varies over the extent ofthe layer.
 6. An apparatus according to claim 1, wherein saidmulti-layer structure is curved.
 7. An apparatus according to claim 1,wherein said multi-layer structure is disposed on the surface of asubstrate.
 8. An apparatus according to claim 7, wherein said substrateis curved.
 9. An apparatus according to claim 1, wherein the thicknessesof the individual layers of the multi-layer structure are the same. 10.An apparatus according to claim 1, wherein the thicknesses of theindividual layers of the multi-layer structures are different.
 11. Anapparatus according to claim 1, wherein said layer has a thickness of 1to 20 nm.
 12. An apparatus according to claim 11, wherein said layer hasa thickness of about 8 nm.
 13. A method of analyzing atomic andmolecular elements by a wavelength dispersive x-ray spectrometric devicecomprising a mirror or focussing arrangement including a multi-layerstructure consisting of a plurality of layer pairs and a measuring andanalysis detector, said method comprising the steps of directing aprimary x-ray or electron beam onto a sample to be tested to inducefluorescence radiation which is then emitted therefrom, directing saidfluorescence radiation onto said mirror or focussing structure at anangle such that said fluorescence radiation is reflected therefrom by atleast one layer pair of said multi-layer structure, of which a firstlayer consists of one of lanthanum and the second of one of a carbon,onto a detector for the analysis of the sample.